1. Field of the Invention
The present invention relates to an X-ray CT (Computerized Tomography) apparatus for industrial purpose to obtain a tomogram of an industrial product such as an electronic component so as to investigate the inner defect, inner structure etc. of the industrial product in a non-destructive manner.
2. Description of the Related Art
According to the X-ray CT apparatus for industrial purpose, in general, as schematically shown in a front view of FIG. 11A and a plan view of FIG. 11B, a rotary stage 53, which rotates around a rotation axis R that is orthogonal to an optical axis L of an X-ray, is disposed between an X-ray source 51 and an X-ray detector 52. X-ray transmission data is obtained from the X-ray detector 52 for each rotation by a predetermined microscopic angle of the rotary stage 53, while irradiating the X-ray on a subject W in a state of holding the subject on the rotary stage 53. A tomogram of the subject W along a plane orthogonal to the rotation axis R of the rotary stage 53 is reconstructed by using the obtained X-ray transmission data (for example, refer to JP-A-2004-117024). Usually, a moving mechanism 54 can move the rotary stage 53 in a direction along the optical axis L of the X-ray (x-axis direction) and directions of y-axis and z-axis that is orthogonal to the x-axis direction.
As methods each for obtaining X-ray transmission data so as to obtain a tomogram of a subject by using such an X-ray CT apparatus, as shown by a schematic plan view in FIG. 12, there region normal scan method and an offset scan method. In a normal scan method, the rotation center ON of a subject W is positioned almost on an optical axis L of an X-ray. In an offset scan method, the rotation center Oo of a subject W′ is shifted by a predetermined distance from the optical axis L of the X-ray (for example, refer to JP-A-2004-117024). Although the offset scan method is deteriorated in a SN (Signal to Noise) ratio as compared with the normal scan method, the offset scan method has an advantage that a field angle of CT imaging can be broadened and a tomogram with a high resolution can be obtained.
In the X-ray CT apparatus of the related art, in general, the positioning of a subject and the change of a magnification are performed while looking at the X-ray transparent image of the subject. That is, of course, in order to obtain an intended tomogram, it is necessary to obtain X-ray transmission data of all points on the tomogram. However, in order to obtain an entire tomogram at a predetermined position (height) of a subject, for example, according to the normal scan method, the subject is required to be disposed at such a position that the subject W is entirely irradiated with an X-ray constantly at the height to obtain the tomogram while the subject W makes one revolution, as shown by the subject W in FIG. 12. According to the offset scan method, as shown by the subject W′ in FIG. 12, the subject W′ is required to be disposed at such a position that there is no region within the subject W′ where the X-ray is not irradiated while the subject W′ makes one revolution. However, there arise a case where the aforesaid condition can not be satisfied, depending on an imaging magnification based on the positional relation in the x-axis (the optical axis L of the X-ray) direction of the rotary stage 53 and the X-ray detector 52 with respect to the X-ray source 51.
Thus, in the X-ray CT apparatus of the related art, before obtaining the X-ray transmission data of a subject, it is required to place the subject on the rotary stage 53 and rotate the subject by one revolution while irradiating the X-ray on the subject. Further, it is required to confirm whether or not the subject is placed on a position satisfying the aforesaid condition based on X-ray transparent images obtained at respective time points. Further, it is required to change the position of the rotating stage 53 and/or the position of the X-ray detector 52 in the x-axis direction when it is determined that the aforesaid condition is not satisfied. In this manner, the X-ray CT apparatus of the related art has a problem that such a confirmation procedure is troublesome.